Polyurethanes are well known and the basic technology for their manufacture is well established. It is well known, that various polyols may be used in making polyurethanes, such as polyesters, polyethers, polyesters amides, and the like. Scant attention has been given to the use of polyformals as the polyol intermediate for polyurethanes.
Polyformals are a special case of polyacetals represented by the following formula where R.sup.1 is hydrogen and n is the degree of polymerization. ##EQU1## They are condensation products of an .alpha.,.omega.-glycol and formaldehyde using an acidic condensation catalyst.
British Pat. No. 850,178, Hudson Foam Plastics Corp., describes a method for making polyformals, with a molecular weight of at least 1270, terminal hydroxyl groups and a hydroxyl number of less than 200, by reacting various diols with formaldehyde in the presence of an acidic catalyst at a temperature not exceeding 130.degree.C. They are described as useful for polyurethane elastomers.
Muller et al in U.S. Pat. No. 2,961,428 discloses polyurethane plastics prepared from hydroxyl terminated polyformals derived from aromatic polyhydroxy compounds with aliphatically bonded hydroxyl groups. They are made in a manner similar to that disclosed in the British patent.
It has been observed that when polyformals are prepared in a conventional manner, as described in the British patent, and reacted with an organic diisocyanate, one or more of the following phenomena may occur: (1) foaming, (2) a higher viscosity is obtained than expected, (3) the isocyanate content of the resulting prepolymer is lower than expected, (4) color develops, and (5) gelation often occurs. In addition, the physical properties of a polyurethane elastomer prepared from such polyformals are inferior having a low hardness. Thus, polyformals have not been used to any significant extent in preparing polyurethane elastomers.
Finally, Schonfeld, J. Poly, Sci. 59, 87-92 (1962) has prepared and studied the properties of a homologous series of polyformal polyurethanes, and has observed that two types of hydroxyl end-groups are possible in polyformals prepared by conventional procedures: (1) methylol or hemiacetal, --O--R--O--CH.sub.2 --OH, and (2) alcoholic, --O--R--OH. Experience has shown that reaction of a diisocyanate with polyols containing alcoholic end-groups, e.g. polyesters, polyethers, polyesteramides, etc., proceeds without undesirable results and ultimately provides polyurethanes with outstanding physical properties. Thus, theoretically at least, it would be expected that polyformals would provide similar results.
The present invention represents the culmination of a long series of investigations, conducted largely by the inventors, directed to overcoming the inherent deficiencies of polyformals.
Accordingly, it is a primary object of the present invention to provide new and improved polyformals.
Another object of the invention is to provide an improved isocyanate terminated polyformal.
It is yet another object of the invention to provide an improved polyurethane elastomer based on the improved polyformals of the present invention.
Generally then, it is an object of this invention to provide an improved polyformal which is capable of functioning to provide polyurethane elastomers with outstanding physical properties.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.